This invention relates to insulated or covered vibration resistant overhead electrical cables. More particularly, it relates to a high-voltage transmission line which is resistant to aeolian vibrations and galloping, and which has no dielectric limitations. In addition, it relates to a cable which can be advantageously installed in a high-voltage transmission line designed to have a low electromagnetic field (EMF).
Aeolian and galloping vibrations of overhead, electrical transmission lines are well known. One known manner of reducing such vibrations is to use a plurality of conductors at least one of which is continuously and helically wound about another conductor so as to provide the final cable with a transverse cross-section which is oval or elliptical in shape and which has a continuously varying profile along the cable""s length. Such conductors are disclosed, for example, in U.S. Pat. No. 3,659,038 of Apr. 25, 1972 where the phenomenon of aeolian vibration is also discussed and the galloping vibration is mentioned. Normally such cables are xe2x80x9cbarexe2x80x9d or xe2x80x9cair-insulatedxe2x80x9d, although in some cases individual conductors may be insulated.
There are also a number of patents which disclose various damping accessories that are attached or clamped onto the cable in order to eliminate vibrations or reduce their amplitude. One such vibration damper is disclosed in U.S. Pat. No. 3,992,566 dated Nov. 16, 1976, and consists of an elongated plastic plate clamped onto the aerial conductor.
All of these prior art cables have several disadvantages.
The xe2x80x9cbarexe2x80x9d or uninsulated conductors are not suitable for low EMF use at transmission line voltages of, for example, 69 kV or higher. A typical high voltage transmission line will have several hundred kilovolts, e.g. 230 kV, and spacings between conductors of 7 to 10 meters. With an insulating layer on the conductors, the interphase spacing can be reduced to 1.5-2 meters. This has the effect of a significant reduction in the electromagnetic radiation (EMP), which varies logarithmically with the average conductor spacing. Extremely low frequency electromagnetic fields are generally defined as those electromagnetic fields of less than 300 Hz. and are believed by some researchers to be cancer facilitators, especially in children. Although several recent epidemiological studies have proven inconclusive, biological effects have been demonstrated under both in vivo and in vitro experimental conditions. Because there is no firm link between exposure to low frequency electromagnetic fields and damage to human health, the short term future individual response may be one of prudent avoidance. Utilities will follow this principle by minimizing electromagnetic fields as much as possible.
However, when a number of insulated conductors are wound around each other, there is produced a dielectric disadvantage of having an assembly of conductors operating at high voltage, leading to less equally distributed dielectric stress within the conductor insulation. This unequal stress distribution will be exacerbated where a fault condition occurs involving only one of the assembled conductors, thereby producing a possibility of exceeding acceptable cable design limits.
The various damper attachments do not alleviate the above disadvantages, but rather produce some of their own such as higher installation and maintenance costs and the like.
It is, therefore, an object of the present invention to provide a vibration resistant overhead conductor which will alleviate the above disadvantages and which will be suitable for low-EMF applications.
Another object is to provide a simple and effective high-voltage overhead cable construction which will resist both aeolian and galloping vibrations.
Other objects and advantages of the invention will be apparent from the following description thereof.
In general, this invention provides a vibration resistant overhead electrical cable comprising an insulated conductor in which the insulation has an axially continuously rotating oval or elliptical outer periphery such that the aerodynamic forces acting on the cable act in a continuously changing direction, thereby reducing the tendengy of the cable to vibrate. The invention covers any overhead electrical cable construction provided it has an insulation overcoating a conductor and having the required outer shape and rotation or twist, however, it is particularly suitable for high-voltage transmission lines with low EHF. Normally such cables have a stranded conductor, which may be a conventional round conductor, with a layer of semi-conducting material provided thereover and acting as a conductor shield. Such conductor shields are well known in power cables and they are normally made of a material having electrical properties which are suitable for this purpose. Then, preferably, two layers of insulation are provided on top of the conductor shield, an inner insulating layer and an outer insulating layer. Obviously, if desired, additional insulation layers or other structural elements of the cable could also be provided.
When two layers of insulation are provided as mentioned above, then the inner insulating layer can be made to have essentially the same shape as the conductor, for example, round, whereas the outer insulating layer is made to have the axially continuously rotating oval or elliptical outer periphery. This can be achieved by applying the outer layer either in a separate or in the same manufacturing process so that it would have this desired rotation and an oval or elliptical outer shape. For this purpose, an oval or elliptically shaped extrusion die, which rotates at such a rate as to obtain the desired pitch of rotation or as it is sometimes called xe2x80x9clayxe2x80x9d, can be advantageously employed.
In some cases it is advantageous to make both the inner insulating layer and the outer insulating layer of oval or elliptical shape. This results in improved dielectric properties of the cable, because, at prevailing operating temperatures, the inner insulation will typically have a lower dielectric constant and be more dielectrically stable than the outer insulating layer.
Moreover, the conductor itself can be made oval or elliptical with a spiralled twist providing the desired longitudinal rotation of the major axis of the cross-sectional shape. On top of such conductor one can apply the layer of semi-conducting material and the desired layers of insulation so that they will all retain the rotating oval or elliptical shape of the conductor and provide the outer periphery of the cable with the desired shape and lay.
Although one can use, in accordance with the present invention, any oval or elliptical geometry of the insulation that will cause, in conjunction with the continuously rotating shape thereof, the wind-induced aerodynamic forces acting on the overhead cable to be in a continuously changing direction, thereby reducing the tendency of the cable to oscillate, it has been determined by experimental analysis that best results are achieved when the oval or elliptical major to minor axis ratio in between 1.1 and 1.2. The length of rotation or lay is usually kept within a range most suitable for manufacturing, however, normally it will be between about 2.5 and 3.5 meters, for example 3 meters.
The outer insulation should normally be made of a material which is weather resistant and also resistant to electrical discharge. It is made of a typically UVxe2x80x94and track-resistant polymer. Such insulating materials are well known in the art of cablemaking.